The present invention relates, in general, to low NO.sub.x burners in large utility boilers, and in particular, to a new and useful cell burner arrangement which uses twin burner cells at the corners of a furnace.
One category of prior art concerns The Babcock & Wilcox Company's (B&W's) pulverized coal cell burners. Cell burners were used on B&W Universal Pressure (UP) Utility Boilers in the 1960's and 1970's as a compact burner design capable of high heat inputs and high combustion efficiency. The closely-spaced paired burner throats (typically on 4'6" vertical center-line spacings) known as cells provide high turbulence and rapid mixing between the pulverized coal and secondary air resulting in relatively high NO.sub.x generation. Cell burners produce little air resistance and poor air distribution between the cells resulting in localized reducing zones in the furnace which leads to furnace wall tube corrosion and slagging problems. Often the air registers are fixed in position, and the airflow patterns within the open, wraparound windboxes tend to feed more secondary air to the burners at the center of the unit, and less air to those cells adjacent to the sidewalls. Cell burners for the largest UP boilers are arranged in two rows on each of the front and rear walls, and are 10 to 14 burner columns wide.
A second category of prior art concerns B&W's S-type burner or any other type of single secondary air zone burner. The S-type burners is a single secondary air zone burner, and its adjustable sliding air damper was designed to facilitate balancing of secondary air flow in multiple burner, open windbox applications. Although not designed to meet EPA NO.sub.x emission limitations, this burner gives the operator the tools to evenly distribute secondary air flow across all the burners of the boiler to minimize furnace waterwall corrosion potential as well as furnace slagging problems. The convertible S-burner was developed as a variation of the S-burner concept, with design features intended to simplify and reduce the cost of conversion to LNCB.RTM. technology in the future.
A third category of prior art concerns B&W's DRB-XCL.RTM. burner or any other type of dual air zone burner designed for internal air staging with the intent of lowering NO.sub.x emissions. The DRB-XCL burner is designed with an adjustable sliding air damper for secondary air flow balancing in multiple burner, open windbox applications. Other dual air zone burner designs have secondary air flow control dampers or registers to facilitate air flow balancing between the burners.
A fourth category of prior art concerns B&W's Low No.sub.x Cell burner (LNCB.RTM.) technology as embodied in B&W's U.S. Pat. No. 5,205,226 which is incorporated herein by reference. This patent covers a low NO.sub.x burner system using an arrangement of B&W's S-burners with enlarged coal nozzles and integral louver-type NO.sub.x ports in inverted and non-inverted arrangements. This LNCB.RTM. equipment is designed as a "plug-in" retrofit which fits into the existing cell throat openings. The various arrangement patterns described in the patent were intended to reduce carbon monoxide (CO) and hydrogen sulfide (H.sub.2 S) concentrations in the lower hopper and burner zone regions of the furnace while maintaining low NO.sub.x emissions. Both the high fuel input S-burner and close coupled NO.sub.x ports are equipped with sliding air dampers for secondary air flow balancing between each throat opening across the boiler within the open windbox.
The subject matter of B&W's Low-NO.sub.x Cell burner disclosed in U.S. Pat. No. 5,205,226 has been cited as "one of the most technologically significant new products of the year" and was selected to receive the coveted 1994 R&D 100 Award. Sharing the award with B&W were the Electric Power Research Institute, the Department of Energy (DOE), the Ohio Coal Development Office within the Ohio Department of Development, and the Dayton Power & Light Company.
The arrangements described in the patent minimize the areas of the furnace waterwalls which are exposed to reducing combustion gases. However, the inherent operation of the high input S-burners at sub-stoichiometric conditions leave the potential for reducing furnace gas conditions along the sidewalls at the burner elevations and above until the secondary air from the close coupled NO.sub.x port oxidizes these furnace gases. This could potentially lead to localized elevated corrosion rates in these waterwall areas requiring some form of corrosion protection, including but not limited to field applied corrosion-resistant coatings. The LNCB.RTM. technology was developed as a plug-in low NO.sub.x solution for cell equipped boilers without resorting to pressure part modifications.
A fifth category of prior art concerns the B&W pulverized coal fired, cell burner equipped universal pressure (UP) boiler. In particular, the 1100 MW to 1300 MW class of supercritical pressure UP boilers with 10, 11, 12, or 14 burner columns across the width of the unit. This type of boiler is the focus of the present invention.
Two 1100 MW units at Duke Power's Belews Creek Units 1 & 2 (B&W designation UP-95 and UP-96), are currently equipped with convertible S-burners and are 10 burner columns wide (40 cells). Two 1300 MW units at TVA's Cumberland Units 1 & 2 (B&W designation UP-73 and UP-81), still have the original cell burners and are 11 burner columns wide (44 cells). Two 1300 MW units at AEP's Gavin Units 1 & 2 (B&W designation UP-102 and UP-107), still have the original cell burners and are 14 burner columns wide (56 cells). One 1300 MW unit at AEP's Amos Unit 3 (B&W designation UP-101), still has the original cell burners and is 12 burner columns wide (48 cells).
The higher operating pressures and temperatures of the supercritical fluid in the lower furnace first and second pass waterwall circuits, produce high furnace waterwall tube surface metal temperatures. Corrosion studies have shown that corrosion rates increase with increasing metal temperature. Therefore, supercritical UP boilers are more susceptible to furnace waterwall corrosion than drum type boilers equipped with the same furnace tube material and exposed to the same combustion environment.